1. Field of the Invention
The present invention relates to an ink-jet printer for ejecting ink droplets through a droplet outlet orifice (a nozzle) and recording an image on paper and an apparatus and a method of driving a recording head for an ink-jet printer.
2. Description of the Related Art
Ink-jet printers for ejecting ink droplets through a droplet outlet orifice communicating with an ink chamber and recording on paper have been widely used. In such an ink-jet printer of related art, a single piezoelectric element is provided for each nozzle. The piezoelectric element is fixed to an oscillation plate forming an external wall of the ink chamber to which ink is fed through an ink duct. The piezoelectric element changes the ink chamber volume by bending in response to a voltage waveform of an applied drive signal so as to generate an ejection pressure. An ink droplet is ejected through the outlet orifice by the ejection pressure.
Since the ejection pressure is generated by changing the ink chamber in such an ink-jet printer as described above, ink ejected through the orifice flies in a columnar shape (in a trailing form). Differences in time and velocity result between the tip and the end of the flying ink droplet.
Consequently, the preceding main ink droplet is accompanied by unwanted minute droplets (called satellite droplets in the following description). Such satellite droplets landing on paper affect the printing result. Although satellite droplets do not have a great effect on the quality of a high-density image recorded with relatively large droplets, the image quality is expected to be significantly reduced by satellite droplets when the image is recorded with small droplets for representing a low-density image or a half-tone image. Satellite droplets generated when small droplets are ejected therefore cause a great problem.
Some methods have been proposed in order to cope with the problem. For example, a method is disclosed in Japanese Patent Application Laid-open Hei 7-76087 (1995) wherein a single piezoelectric element is provided for each nozzle and the velocity of changing ejection voltage applied to the piezoelectric element is switched between two levels for ejecting ink droplets. In the method, as shown in FIG. 1, the ejection voltage is initially increased at first voltage changing velocity xe2x80x98v1xe2x80x99. The ejection voltage is then increased at second voltage changing velocity xe2x80x98v2xe2x80x99 higher than v1. In FIG. 1, the vertical axis indicates voltage. The horizontal axis indicates time. According to the method, the next droplet is ejected to follow the tip of the preceding droplet. The difference in velocity between the tip and the end of the ink column is thereby decreased and satellite droplets are reduced.
Another method is disclosed in Japanese Patent Application Laid-open Sho 59-133067 (1984) wherein a single piezoelectric element is provided for each nozzle and an ink droplet is ejected by applying two independent voltage pulses to the piezoelectric element. In the method, as shown in FIG. 2, first pulse P1 is applied to the piezoelectric element to produce a first pressure fluctuation for starting ink droplet ejection through a nozzle. First pulse P1 is then terminated and second pulse P2 is applied to the piezoelectric element before the ejection of droplet through the nozzle is completed to produce a second pressure fluctuation. In FIG. 2, the vertical axis indicates voltage. The horizontal axis indicates time. According to the method, the ink column ejected through the nozzle ruptures at an early stage and generation of satellite droplets is suppressed.
An ink droplet ejection apparatus is disclosed in Japanese Patent Application Laid-open Sho 51-45931 (1976) wherein two pressure generating means are provided for each nozzle and an ink droplet is ejected by oscillating ink by combining oscillations produced by the two pressure generating means.
In the method disclosed in Japanese Patent Application Laid-open Hei 7-76087 (1995) described above, however, first voltage changing velocity v1 is required to be lower than second voltage changing velocity v2. Consequently, the velocity of an ejected ink droplet is reduced when compared to the case wherein the voltage is changed at high velocity v2 throughout the ejection cycle. A reduction in velocity of an ejected ink droplet results in unstable ejection affecting linearity of the droplet flying route and variations in droplet velocity. As a result, displacements of recorded dots may occur and printing quality may be reduced.
In the method disclosed in Japanese Patent Application Laid-open Sho 59-133067 (1984) described above, second pulse P2 is applied after interval Ti, having terminated first pulse P1. If interval Ti is too long, a trail of an ink column becomes long and satellite droplets may be produced. On the other hand, if interval Ti is too short, the piezoelectric element does not follow the voltage change and the intended operation will not be achieved. This is because the piezoelectric element in general has its intrinsic oscillation characteristic and does not operate at a frequency above the intrinsic oscillation. Although this problem may be solved by fabricating a piezoelectric element having a high intrinsic frequency, this is not realistic since there is a limitation of the intrinsic frequency of the piezoelectric element obtained in practice. In addition, fabricating such a piezoelectric element is accompanied by technical difficulties and manufacturing costs are thereby increased. Furthermore, in the above-mentioned publication, although voltage V1 of first pulse P1 is lower than voltage V2 of second pulse P2, voltage V1 is required to be higher than voltage V2 so that the trailing end of the ink column reaches the tip thereof and becomes integrated with the tip. However, an increase in the voltage applied to the piezoelectric element causes a reduction in the life of the piezoelectric element and the oscillation plate oscillated by the piezoelectric element. A residual oscillation is increased as well and the frequency characteristic may be affected.
The above-mentioned ink droplet ejection apparatus disclosed in Japanese Patent Application Laid-open Sho 51-45931 (1976) is provided for efficiently ejecting ink droplets with a small power input. In order to achieve the object, high-frequency drive signals are each applied to the two pressure generating means and the phase difference between the drive signals and the amplitude are changed so that the oscillations generated by the pressure generating means are successfully combined to oscillate ink. An ink droplet is thereby ejected. That is, the apparatus is not intended for preventing satellite droplets. The method of driving the pressure generating means and the configuration required for preventing satellite droplets are not disclosed, either. No suggestion about such a method or configuration is made in the publication, either.
As thus described, it is difficult to satisfactorily reduce satellite droplets in the related art without reductions in velocity of an ejected droplet, in the apparatus life, in the frequency characteristic and without a limitation of the intrinsic oscillation characteristic of the piezoelectric element.
The related-art ink-jet printers have further problems. FIG. 3 is a schematic diagram of a recording head and a drive circuit thereof in a related-art ink-jet printer. As shown, a recording head 500 includes a nozzle 501 and a piezoelectric element 502 provided in correspondence with the nozzle 501. The piezoelectric element 502 is fixed to a wall of an ink chamber (not shown) to which ink is supplied through an ink duct (not shown). A drive signal 504 of a specific waveform is selectively inputted to the piezoelectric element 502 through an on/off switch 503. That is, the drive signal 504 is only inputted to the piezoelectric element 502 when the switch 503 is turned on. On the application of the drive signal 504, the piezoelectric element 502 is bent in such a direction that the ink chamber volume is reduced. An ink droplet is thereby ejected through the nozzle 501.
For such printers, one of the methods for producing halftone images is varying a droplet size dot by dot. In the drive circuit of the recording head of related art shown in FIG. 3, however, only one type of drive signal 504 is inputted so that merely whether to perform ejection or not is the only operation that is controlled. Consequently, it is impossible to perform control for varying a size of ejected droplet from droplet to droplet although the interval between recorded dots is controlled. It is therefore difficult to faithfully achieve various image representations such as more natural halftone images.
It is an object of the invention to provide an ink-jet printer and an apparatus and a method of driving a recording head for an ink-jet printer for suppressing generation of satellite droplets accompanying an ejected ink droplet while overcoming the problems described above.
An ink-jet printer of the invention comprises: a droplet outlet orifice through which an ink droplet is ejected; an ink chamber for supplying ink to the outlet orifice; a first pressure generating means for generating a pressure for having the ink droplet ejected through the outlet orifice by changing the volume of the ink chamber through displacement; a second pressure generating means for generating a pressure for suppressing generation of minute ink droplets accompanying the ink droplet ejected through the outlet orifice by changing the volume of the ink chamber through displacement; and an ejection control means for controlling a state of the displacements of the first and second pressure generating means.
An apparatus of the invention is provided for driving a recording head for an ink-jet printer including a droplet outlet orifice through which an ink droplet is ejected; an ink chamber for supplying ink to the outlet orifice; a first pressure generating means for generating a pressure for having the ink droplet ejected through the outlet orifice by changing the volume of the ink chamber through displacement; a second pressure generating means for generating a pressure for suppressing generation of minute ink droplets accompanying the ink droplet ejected through the outlet orifice by changing the volume of the ink chamber through displacement. The apparatus comprises: a means for generating drive signals for effecting the displacements of the first and second pressure generating means; and a means for controlling a state of supplying the drive signals to the first and second pressure generating means.
A method of the invention is provided for driving a recording head for an ink-jet printer including a droplet outlet orifice through which an ink droplet is ejected; an ink chamber for supplying ink to the outlet orifice; first and second pressure generating means provided for the outlet orifice. The method comprises the steps of: generating an ejection pressure for having the ink droplet ejected through the outlet orifice by changing the volume of the ink chamber through displacement of the first pressure generating means by applying drive signals for ejection having a specific waveform to the first pressure generating means; and generating an auxiliary pressure for suppressing generation of minute ink droplets accompanying the ink droplet ejected through the outlet orifice by changing the volume of the ink chamber through displacement of the second pressure generating means by applying an auxiliary drive signal having a specific waveform to the second pressure generating means. A state of the generation of the ejection pressure and a state of the generation of the auxiliary pressure are controlled.
According to the ink-jet printer and the apparatus and method of driving a recording head for an ink-jet printer of the invention, the first and second pressure generating means are provided for the outlet orifice. A state of the displacements of the first and second pressure generating means is adjusted. The auxiliary pressure generated by the displacement of the second pressure generating means is superimposed on the ejection pressure generated by the displacement of the first pressure generating means. Trailing of ink droplet is thereby cut off at an early stage.
Another ink-jet printer of the invention comprises: a droplet outlet orifice through which an ink droplet is ejected; an ink chamber, having a wall, for supplying ink to the outlet orifice; a first pressure generating means provided on the wall of the ink chamber for generating a pressure for having the ink droplet ejected through the outlet orifice by changing the volume of the ink chamber through displacement; a second pressure generating means provided on the wall of the ink chamber for generating a pressure for assisting the ejection of the ink droplet through the outlet orifice by changing the volume of the ink chamber through displacement. The first pressure generating means is placed further from the droplet outlet orifice than the second pressure generating means. xe2x80x98Assisting with the ejection of the ink dropletxe2x80x99 means that adjustment is made so that the ink droplet is ejected in an intended state. To be specific, a specific modification is made on the ejection pressure generated by the first pressure generating means so that the ejected droplet has an intended size and velocity or no unwanted droplet is ejected. The same applies to the following description. For example, the second pressure generating means may generate a pressure for suppressing generation of minute ink droplets accompanying the ink droplet ejected.
Another apparatus of the invention is provided for driving a recording head for an ink-jet printer including a droplet outlet orifice through which an ink droplet is ejected; an ink chamber, having a wall, for supplying ink to the outlet orifice; a first pressure generating means provided on the wall of the ink chamber for generating a pressure by changing the volume of the ink chamber through displacement; and a second pressure generating means provided on the wall of the ink chamber for generating a pressure by changing the volume of the ink chamber through displacement. The first pressure generating means is placed further from the droplet outlet orifice than the second pressure generating means. The apparatus comprises: a means for generating a main drive signal for having the first pressure generating means generate a pressure for ejecting the ink droplet through the outlet orifice and an auxiliary drive signal for having the second pressure generating means generate a pressure for assisting the ejection of the ink droplet through the outlet orifice; and a control means for performing control such that the main drive signal and the auxiliary drive signal are each supplied to the first pressure generating means and the second pressure generating means. The auxiliary drive signal may be the signal generating a pressure for suppressing generation of minute ink droplets accompanying the ink droplet.
Another method of the invention is provided for driving a recording head for an ink-jet printer including a droplet outlet orifice through which an ink droplet is ejected; an ink chamber, having a wall, for supplying ink to the outlet orifice; a first pressure generating means provided on the wall of the ink chamber for generating a pressure by changing the volume of the ink chamber through displacement; and a second pressure generating means provided on the wall of the ink chamber for generating a pressure by changing the volume of the ink chamber through displacement. The first pressure generating means is placed further from the droplet outlet orifice than the second pressure generating means. The method comprises the steps of: applying a main drive signal to the first pressure generating means for generating a pressure for ejecting the ink droplet through the outlet orifice; and applying an auxiliary drive signal to the second pressure generating means for generating a pressure for assisting the ejection of the ink droplet through the outlet orifice.
According to the ink-jet printer of the invention, the first pressure generating means is provided on the wall of the ink chamber in the position away from the outlet orifice. The volume of the ink chamber is changed by the displacement of the first pressure generating means and a pressure is generated for having the ink droplet ejected through the orifice. The second pressure generating means is provided on the wall of the ink chamber in the position closer to the outlet orifice the first pressure generating means. The volume of the ink chamber is changed by the displacement of the second pressure generating means and a pressure is generated for assisting the droplet ejection.
According to the apparatus and method of driving a recording head for an ink-jet printer of the invention, the main drive signal is applied to the first pressure generating means provided on the wall of the ink chamber in the position away from the outlet orifice for generating a pressure for ejecting the ink droplet through the orifice. The auxiliary signal is applied to the second pressure generating means provided on the wall of the ink chamber in the position closer to the outlet orifice for generating a pressure for assisting the droplet ejection. The droplet ejection is thereby controlled.
Still another ink-jet printer of the invention comprises: a droplet outlet orifice through which an ink droplet is ejected; a plurality of energy generating means each for generating energy for having the ink droplet ejected through the outlet orifice; and a plurality of selection means each provided for the respective energy generating means for selecting any of a plurality of drive signals for driving the energy generating means and supplying the signal to the respective energy generating means.
Still another apparatus of the invention is provided for driving a recording head for an ink-jet printer including a droplet outlet orifice through which an ink droplet is ejected; and a plurality of energy generating means each for generating energy for having the ink droplet ejected through the outlet orifice. The apparatus comprises: a means for generating a plurality of drive signals for driving the energy generating means; and a plurality of selection means each provided for the respective energy generating means for selecting any of the drive signals and supplying the signal to the respective energy generating means.
Still another method of the invention is provided for driving a recording head for an ink-jet printer including a droplet outlet orifice through which an ink droplet is ejected; and a plurality of energy generating means each for generating energy for having the ink droplet ejected through the outlet orifice. The method comprises the steps of: selecting any of a plurality of drive signals for driving the energy generating means for each of the energy generating means; and supplying the selected drive signal to the respective energy generating means.
According to the ink-jet printer and the apparatus and method of driving a recording head for an ink-jet printer of the invention, one of the drive signals is selected and supplied to each of the plurality of energy generating means. An ink droplet is ejected through the orifice with the drive signal.
Other and further objects, features and advantages of the invention will appear more fully from the following description.